Modern lithography tools require extremely accurate placement of the lithographic substrate (e.g., a semiconductor wafer) relative to the imaging optical system to ensure optimum focus of the images formed on the substrate surface by the imaging optical system. Also, current demands on throughput require that any focus adjustments from one exposure to the next be performed extremely rapidly (and hence automatically).
During manufacture of microelectronic devices, multiple lithographic steps are required to form the multiple circuit-defining, thin-film layers of the devices. The various layers thus formed typically have different patterns of circuit elements. Autofocus (AF) devices in current lithography tools are usually located in the vicinity of the substrate and imaging optical system and usually involve directing a beam of light (called AF light) at a region of the substrate at a glancing angle of incidence. The AF light can be, for example, an image of a slit or a set of interference fringes. The AF light reflected from the substrate surface is electronically detected and analyzed, and the resulting data are used for positioning the substrate relative to the imaging optical system. AF devices can work in conjunction with other devices for detecting and adjusting substrate “height” relative to the imaging optical system.
AF devices can produce erroneous substrate-height data due to thin-film effects occurring in the multiple layers of material already formed on the substrates. In other words, the devices can be adversely affected by the optical properties of the substrate surface and underlying structures, such as layers (usually but not necessarily patterned layers) already formed on the substrate during earlier process steps. Light reflected from the substrate frequently exhibits changes in intensity and/or phase that are unrelated to actual substrate height. An AF device may produce a height estimate including errors related to these changes, particularly changes that are non-linearly related to the thicknesses and refractive indices of the underlying layers. These errors are termed “AF errors” or “focusing errors.”
An important source of AF errors arises from some of the AF light (which may form images of “slits” or “fringes” on the substrate) reflecting from previously formed patterned thin films beneath the surface of the substrate. The magnitude of AF errors of this general type can vary with the particular pattern(s) and other features in the previously formed layers on the substrate, and can vary with the thickness profiles of those layers. These AF errors can be substantial.
An example AF error arising from the presence of thin-film layers previously formed on the substrate is the Goos-Hanchen (GH) effect, in which a substrate surface including thin films formed during earlier process steps produces a shift or offset in position of a beam of light reflected from the surface. This shift is not related to an actual change in position of the substrate, but can be mistaken for one. Patterning in previously formed thin films is not required for the GH effect to occur (but patterning can be a factor). Rather, the simple presence of the previously formed thin films is required. If the substrate has the same thin-film stack applied uniformly over the substrate surface, then the result usually is a substantially uniform GH effect and produces a uniform offset that is easily treated by introducing an offset from the measured substrate height. Otherwise, GH effects can vary appreciably over the substrate surface, depending upon regional variations of thickness and other parameters of the thin films over the surface as well as pattern variations from one region to the next. For example, a change in regional offset due to the GH effect can arise in a substrate area containing memory circuits relative to another area containing logic circuits. Also, different patterns, although they may consist of the same material, may produce different offsets from GH effects by virtue of the structures, spatial frequencies, orientation, duty cycles, etc., of the respective patterns relative to other patterns.
In producing a GH effect, previously formed layers on the substrate surface can change the intensity of reflected AF light and/or the phase of that light. According to one way of looking at the GH effect, a monochromatic AF beam incident on a reflecting surface can be decomposed into multiple plane waves. The reflective surface (i.e., the substrate surface) produces a different phase for each of these plane waves, depending on the wave's angle of incidence. Over a small range of incidence angles (corresponding to a converging or diverging wavefront), the phase of reflection can either increase or decrease with the angle of incidence, which produces a tilted wavefront in the far field corresponding to a physical shift of the beam in the near field. The GH effect is the apparent shift of the beam, which is manifest as an AF error.
Correction of an AF error caused by the GH effect is called a Goos-Hänchen correction (GHC). An example GHC is discussed in U.S. Patent Publication No. 2011/0071784 (called herein the “'784 reference”), incorporated herein by reference to the fullest extent allowed by law. The '784 reference discusses a fringe-projection AF device in which broadband AF light (having variable wavelength and polarization) is reflected from a substrate and detected along with AF light reflected from a reference mirror. GH effects, assumed to be present, are estimated from measured changes in spectral and polarization properties of the light reflected from the substrate on which a “stack” of previously formed layers has been formed. For example, the spectral and polarization properties may include a change in the change of phase with respect to the angle of incidence (and position on the substrate). The GH estimates thus obtained are used to estimate corresponding GHC's. Another conventional GHC is discussed in U.S. Patent Application Publication No. 2012/0008150, also incorporated herein by reference to the fullest extent allowed by law. Unfortunately, conventional slit- and fringe-projection AF devices still exhibit pattern-dependent phase changes to the AF light.